THE OLDEST FISHES AND THEIE FINS. 143 



CHAPTER XIV. 



THE OLDEST FISHES AND THEIR FINS. 



It is a well-known fact that while the fishes of the more 

 ancient periods of the earth's history were frequently 

 characterized by having their bodies protected by a coat of 

 armour, this armour has been lost by most of their modern 

 descendants. Nevertheless, a few of these mail-clad fishes, 

 like the gar- pike of the rivers of North America, and the 

 mauy-finned bichir (Polypterus) of the upper Nile and the 



Fig. 47.— The Bichir. 



rivers of the West Coast, still linger on, as if for the 

 purpose of showing us what their ancestors were like. 



In addition, however, to their armour, and its gradual 

 loss with the advance of time, there are many other points 

 of view from which these ancient fishes are of more than 

 ordinary interest, and we accordingly propose in this 

 chapter to consider the curious modifications which have 

 taken place in the structure of their fins as we ascend in 

 the geological scale. We shall, moreover, be led to notice 

 briefly one of the most remarkable types of fossil fish 

 teeth found in the older Secondary rocks, since we can 

 thereby prove that one of our living fishes is the oldest 

 kind of vertebrate now inhabiting the earth. 



Before going further, we must mention that existing 

 fishes have been divided into several main groups, dis- 

 tinguished from one another by structural peculiarities. 

 One such grouj) includes the sharks and rays, characterized 

 by their cell-bke gills and scaleless bodies. Then we have 

 the smaller group of lung-fishes, now represented by the 



